The Third Generation Partnership Project (3GPP) is considering the standardization of mobile communication schemes. The 3GPP has standardized for example high-speed wireless communication schemes such as LTE (Long Term Evolution) etc. Further, in 3GPP Release 12, the standardization of D2D communication is in progress as a new wireless communication scheme. D2D communication is one of the extended specifications of LTE, and is sometimes referred to as LTE Device to Device Proximity Services.
In D2D communication, a communication device can directly communicate with another communication device without going via a base station. Therefore, D2D communication is expected to realize communications with low latency. Also, D2D communication can be performed even in an area that radio waves of a base station do not reach (or an area not having a base station), and thus can contribute to the expansion of network coverage. Further, D2D communication can be performed even in a situation where it is not possible to use a base station (such as a case when a massive earthquake occurred for example), and thus can contribute to the improvement of the security of users. Note that a communication link established between communication devices for a D2D communication may also be referred to as a D2D link.
3GPP Release 13 includes a description related to Vehicle-to-Vehicle (V2V) services. V2V services may be implemented by utilizing for example D2D communications. In such a case, a V2V service is realized by a direct communication that is performed, without going via a base station, between a communication device implemented in a vehicle and a communication device implemented in another vehicle. Also, various applications are proposed for V2V services. For example, a vehicle having priority such as an ambulance etc. can make other vehicles traveling around that vehicle recognize the existence of that vehicle. Further, when a traveling vehicle stops urgently, it is possible to make other vehicles traveling around that vehicle recognize the urgent stop.
Communications using communication devices implemented in vehicles are disclosed by for example Japanese Laid-open Patent Publication No. 2004-185428, Japanese Laid-open Patent Publication No. 2005-229478, Japanese Laid-open Patent Publication No. 2009-147652, and Japanese Laid-open Patent Publication No. 2009-253731.
Scheduling for a D2D communication is performed by using an SA (Scheduling Assignment) message. Specifically, communication resources for a D2D communication are divided into SA areas and data areas. An SA area is used by a communication device to report an SA message to another communication device around the communication device. An SA message can indicate a communication resource (such as a time slot, a frequency, etc. for example) for transmitting data. Thereby, a communication device that receives an SA message can extract target data from a wireless signal.
The SA areas are inserted at prescribed time intervals. In the descriptions below, time intervals at which SA areas are inserted may also be referred to as “scheduling periods”. The length of a scheduling period is 40 m seconds through 320 m seconds, although the length depends upon communication modes.
When transmitting data to a destination communication device, a communication device transmits an SA message to the destination communication device before transmitting that data. When for example transmission data is generated at time T1 illustrated in FIG. 1, the communication device uses the next SA area to transmit an SA message to the destination communication device, and thereafter transmits data to the destination communication device in accordance with the SA message. Accordingly, it is not possible for the communication device to transmit data in the period between T1 and T2 in the example illustrated in FIG. 1. In other words, latency depending upon a scheduling period may occur in D2D communication.
Meanwhile, there is a demand that latency be very low in a V2V service in some cases. For example, there is a demand that a pre-crash sensing warning be able to be transmitted with latency of 20 m seconds or shorter. However, it is sometimes difficult to meet this demand of latency in a case when a V2V service is provided by using an existing D2D communication.
A shorter scheduling period may solve this problem. However, it is sometimes not possible to transmit priority data (emergency data in particular) due to a conflict of the allocation of resources even with a shortened scheduling period. When for example a V2V service is provided by using a D2D communication, each communication terminal autonomously selects an arbitrary resource from a resource pool that is prepared in advance. This means that priority data transmitted from a communication device and non-priority data (i.e. normal data) transmitted from another communication device may sometimes collide in a case when a plurality of communication devices select the same resource for a V2V service. This leads to a possibility of failure of a communication of priority data.
Note that these problems arise not only in a V2V service but also in a wireless communication system that directly performs a communication between communication devices.